Selective nanoprobe for olfactory medicine

ABSTRACT

An apparatus is for detecting the presence of selected gaseous compounds indicative of a medical condition includes a probe having one or more sensor elements having a conductivity which is responsive to the presence of selected gaseous compounds, at least one sensor element containing a transition metal oxide selected from the group consisting of WO 3 , beta-MoO 3  and UO 2 ; and means for measuring the conductivity of each said sensor element.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional Application of U.S. patent application Ser. No. 11/351,171, filed in the U.S. Patent and Trademark Office Feb. 9, 2006, which was a Continuation In Part of U.S. patent application Ser. No. 10/419,349, filed Apr. 21, 2003, which are incorporated herein by reference in their entirety.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support of Grant No. SGER DMR0224642 awarded by the National Science Foundation. The Government has certain rights in this invention.

BACKGROUND

1. Field of the Invention

The present application relates to a method and apparatus for selectively detecting gaseous compounds.

2. Background of the Art

There has been an increasing demand for electronic instruments that can mimic the human olfactory processes and which may provide low-cost and rapid sensory information to hasten the process of evaluation of odors. This led to the conceptualization of an electronic nose (“e-nose”) as an intelligent chemical array sensor system for odor classification. At the same time the need for biochemical detectors that are able to sense the presence of pathogens in humans and which can contribute to the early detection of diseases is high.

Although analysis of body fluids (blood, sputum, urine) for disease diagnoses and monitoring is routine clinical practice, human breath analysis methodologies that exploit the non-invasive nature of such diagnoses are still under-developed. Since antiquity, such as the time of Hippocrates, exhaled breath was known to enable non-invasive detection of disease.

Breath testing devices first appear in 1784 when Lavoisier detected CO in the exhaled breath of guinea pigs. Since then, colorimetric assays and gas chromatography columns have been used to detect VOCs (volatile organic compounds) in (human) breath in quantities varying from millimolar (10⁻³M) to picomolar (10⁻¹²M) concentrations. The latter gas sensitivity limit was achieved by Linus Pauling's gas-chromatography-based breath analysis device in 1971. Among the 400 compounds of which the human breath consists, only 30 have been identified by GC/MS.

Various apparatus and methods for olfactometry are disclosed in U.S. Pat. Nos. 6,841,391, 6,839,636, 6,820,012, 6,767,732, 6,703,241, 6,620,109, 6,609,068, 6,606,566, 6,467,333, 6,411,905, and 6,319,724, all of which are incorporated herein by reference. A transition metal oxide gas sensor is disclosed and described in U.S. Pat. No. 6,173,602, which is herein incorporated by reference.

There are various medical uses for “e-nose” sensors. Gas odors evolving from infections or disorders provide the pathway for chemical sensors to be used in bio-chemical applications. NO₂ is a gas associated with bronchial epithelial infection arising out of tobacco smoking. Ammonia is a product of decomposition of urea. Hence, chemical sensors capable of detecting these chemical species can be easily used for medical applications. The key deficiency of current sensor/e-nose technologies is the lack of specificity in recognizing the particular analyte/gas detected.

SUMMARY

An apparatus is provided herein for detecting the presence of selected gaseous compounds indicative of a medical condition. The apparatus comprises a probe having one or more sensor elements having a conductivity which is responsive to the presence of selected gaseous compounds, at least one sensor element containing a transition metal oxide selected from the group consisting of WO₃, beta-MoO₃ and UO₂; and means for measuring the conductivity of each said sensor element.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are described below with reference to the drawings wherein:

FIG. 1 is a graph of sensing data showing the selective response of orthorhombic MoO₃ to ammonia gas;

FIG. 2 is a graph of sensing data of the orthorhombic phase of WO₃ showing its response to NO₂ gas;

FIG. 3 is a chart of XRD spectra of the two phases of WO₃ films used for sensing (W2 represents monoclinic WO₃ films and W3 represents orthorhombic WO₃ films;

FIG. 4 includes graphs of sensing data for the monoclinic phase (W2) and the orthorhombic phase (W3) of the WO₃ films for NO₂ detection at 200° C.;

FIG. 5A is a graph of relative sensitivities of the 5-sensor array to 10 ppm acetone;

FIG. 5B is a graph of relative sensitivities of the 5-sensor array to 10 ppm ethanol;

FIG. 6A is a graph of the response of a monoclinic MoO₃ sensor to 200 ppm NO₂, 200 ppm NH₃ and 10 ppm CO;

FIG. 6B is a graph of the response of a monoclinic WO₃ sensor to 200 ppm NO₂, 200 ppm NH₃ and 10 ppm CO; and

FIG. 7 is a schematic illustration of an apparatus for detecting the presence of target analytes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

The apparatus described herein is an artificial olfactory system capable of detecting selected gaseous compounds, or analytes, in the breath of a mammalian subject. The apparatus includes a probe having at least one sensor, and preferably an array of sensors that selectively produce a measurable response in the presence of the target analyte(s). A unique signature output is produced which can be analyzed by appropriate algorithms to identify the type and amount of the analyte(s) in the breath of the subject human or animal.

Medical studies reported recently have associated certain gaseous constituents of the human breath with specific types of diseases, and have addressed the importance of diet, dental pathology, smoking, etc. on determining the physiological levels of the biomarker concentrations in exhaled breath. Inflammation and oxidative stress can by monitored by measuring the changes in the concentration of, for example: NO; exhaled CO, which is also a marker for cardiovascular diseases, diabetes, nephritis, bilirubin production; exhaled hydrocarbons of low molecular mass, such as ethane, n-pentane; ethylene, isoprene—(isoprene is a hydrocarbon derived from the cholesterol synthesis pathways in humans in vivo, its concentration is affected by diet, and can be used as a specific marker for monitoring blood cholesterol levels by sampling exhaled human breath); acetone which is a marker for diabetes; formaldehyde; ethanol; hydrogen sulfide and carbonyl sulfides indicating liver function impairment; and ammonia/amines—the latter are signaling markers of renal diseases.

The apparatus described herein employs a probe having at least one sensor with a transition metal oxide which has a conductivity selectively changeable in response to the presence of particular analytes. The change in conductivity is believed to result from red-ox reactions occurring at the surface of the oxide. Preferably, an array of sensors with different transition metal oxides is employed in the probe. Each sensor produces a characteristic response to particular analytes. The individual sensor together represent a signature response which can be analyzed by appropriate algorithms to determine the presence and amounts of particular analytes. This data, in turn, provides information about the medical condition of the subject.

In an embodiment of the invention, at least one sensor includes tungsten trioxide, WO₃, as the transition metal oxide. The WO₃ can have a monoclinic or orthorhombic crystal structure. Preferably, additional sensors are employed in the probe to provide an array. Non-limiting examples of other transition metal oxides which can be employed include molybdenum trioxide, MoO₃, which can be orthorhombic (alpha-MoO₃) or monoclinic (beta-MoO₃), titanium dioxide (TiO₂), tin oxide (SnO₂), chromium oxides (CrO₂, Cr₂O₃), iridium oxide (IrO₂) and manganese dioxide (beta-MnO₂), either individually or in physical combination.

The metal oxides used in the present invention are preferably binary metal oxides that may be classified into three groups according to their crystallographic characteristics. The gases detected by these metal oxides are oxidizing or reducing gases similarly fall into three groups.

The first group of metal oxides, classified herein as “rutile structured” metal oxides, possess a rutile structure similar to that found in titanium dioxide (TiO₂). The rutile structure is tetragonal, but in some cases it has been described as a distorted hexagonal close packed oxide array with half the octahedral sites occupied by the metal. Alternative rows of octahedral sites are full and empty. The rutile structure is regarded as an ionic structure. Examples of rutile structured metal oxides include TiO₂, SnO₂, CrO₂, IrO₂ and beta-MnO₂, etc.

The second group of metal oxides, classified herein as “rhenium oxide structured” or ReO₃ type metal oxides, possess a cubic structure similar to that found in rhenium oxide, which is closely related to the structure found in perovskite (CaTiO₃). The unit cell of the crystal contains metal atoms at the corners with oxygen at the center edges. Metal oxides which have this structure include WO₃, beta-MoO₃, and UO₂ and possess a monoclinic crystal structure.

The third group of metal oxides are classified herein as “alpha-MoO₃ type” metal oxides. And have a unique, weakly bonded 2D layered structure. Alpha-MoO₃ is a typical representative of this group and possess an orthorhombic crystal structure.

Gases which may be detected in accordance with the present invention may similarly be placed into three categories. Type I gases are nitrogen-lacking reducing gases including, but not limited to, CO alcohols and hydrocarbons. Type II gases are nitrogen-containing reducing gases including, but not limited to, ammonia and amines. Finally, Type III gases are oxidizing gases including, but not limited to, oxygen, and oxides of nitrogen such as, NO, NO₂, etc.

It has been found that rutile structured metal oxides are selective in their sensitivity to the nitrogen-lacking reducing gases (Type I), the rhenium oxide ReO₃ type metal oxides are selective in their sensitivity to the oxidizing gases (Type III) and the alpha-MoO₃ type metal oxides are selective in their sensitivity to the nitrogen-containing reducing gases (Type II).

The detection process of oxidizing/reducing gases by semiconducting metal oxides involves the change in the conductivity of the oxide in the presence of the gas due to catalytic red-ox reactions occurring at the oxides' surfaces. These catalytic reactions are controlled by the electronic structure of the oxide system used as well as by the chemical composition, crystal structure, and relative orientation of the surfaces of the oxide phase(s) exposed to the gas.

Considering other oxides isostructural with MoO₃, but with higher stability at high temperature exposures, tungsten oxides have been identified as promising candidates. Contrary to what was expected, the orthorhombic phase of WO₃ did not respond to the presence of ammonia but was very sensitive to NO₂ (see FIG. 2). The reason for this apparent discrepancy in the gas sensing properties of the respective polymorphs of these two oxides is believed to lie in their crystallographic differences and the specific sensing mechanism that each favors. Whereas α-MoO₃ has a layered structure with (010) basal plane that is built up of double chains of edge-sharing [MoO₆] octahedra connected through vertices, the orthorhombic WO₃ is structurally similar to the ReO₃ type structure of monoclinic MoO₃ (β-phase). WO₃ is selective to NO₂ and similar behavior has been observed for β-MoO₃ (as shown below). An adsorption (chemisorption) based sensing mechanism is active in this case that does not affect the bonds in the metal oxide surface.

The crystal structure of WO₃ is a distortion of rhenium oxide cubic structure in which tungsten atoms are located in cube corners and the oxygen atoms are located on the cube edges. Each tungsten atom is surrounded by six oxygen atoms forming an octahedron. The slight rotation of these octahedral with respect to each other, as well as unequal bond lengths in the octahedral coordination, causes lattice distortion and reduces the symmetry. The distorted structure is stable in several forms giving rise to different phases depending on the temperature. Thus WO₃ exists in various polymorphic forms. There are at least 7 known polymorphic transformations in the temperature range of 0 to 1220° K. In bulk crystals, the triclinic phase is stable below 17° C., the monoclinic phase is stable between 17-320° C. and the orthorhombic is stable from 320-720° C. above which tetragonal phase is the dominant phase. Stoichiometric WO₃ readily loses oxygen to form WO_(3-x). When oxygen is removed from the lattice in sufficient amounts, some of the octahedral change their character from corner-sharing to edge-sharing. In slightly reduced WO₃, shear planes are introduced in many directions at irregular intervals.

Semiconducting metal oxides show surface sensitivity to redox reactions involving gases. For example, during hydrocarbon oxidation reactions, hydrogen abstraction occurs at (001) and (100) crystal phases of MoO₃ while oxygen addition occurs at the (010) basal plane. The local environment of active sites (available for gas adsorption, hydrogen extraction, or oxygen addition) and the orientation of the surface containing these active sites, vary for different phases of a given system, and thus the catalytic behavior of these phases is variable. Oxygen basicity is found to increase with decreasing Mo—O bond strength occurring at different surfaces. Surface oxygen vacancies formed under reduction conditions may influence gas adsorption processes. They may result in slightly reduced metal oxide surfaces which undergo re-oxidation by gaseous oxygen (adsorption-based sensing) or they may order and give rise to crystallographic shear structures that accommodate non-stoichiometric metal oxide compositions known as Magnéli phases (reaction-based sensing). The presence of ordered vacancies and crystallographic shear structures provides a mechanism for selective oxidation.

Considering the two main polymorphs of MoO₃, the α-phase (orthorhombic) has a layered structure with (010) basal plane which is built up of double chains of edge-sharing [MoO₆] octahedral connected through vertices while the β-phase (monoclinic) has a ReO₃-type structure. The ReO₃ type structure does not contain the Van der Waals gap of the α-MoO₃. Therefore, α-MoO₃ upon reduction in catalysis forms the Mo₁₈O₅₂ structure instead of the ReO₃ type Mo₈O₂₃ shear structure. It was discussed above that the α-phase is selective to ammonia and highly sensitive to amines (which are moderate bases) and the sensing mechanism is consistent with the reduction of MoO₃ and the formation of ordered phases, which suggests reaction-based sensing process. Thus, it seems that ammonia and amines can be best sensed by “loosely bound” layered oxide structures, such as the orthorhombic α-MoO₃ phase that enable the reaction of lattice oxygen with the gas and provide easy mechanisms for accommodating the off-stoichiometric M:O ratio. On the other hand, oxidizing gases, such as NO₂, may be easily adsorbed on ReO₃-type crystals. Oxidizing gases destroy oxygen defects.

CO and hydrocarbons typically react with adsorbed oxygen from the environment and may be sensed by rutile-type structures, such as the polymorphs of SnO₂ and TiO₂. Since a given crystal structure may be sensitive to more than one gases, sensing tests at different temperatures need to be carried out so as to identify the optimum operating temperature for the specific sensor. It is important to remain within the phase stability field of the particular polymorph of the oxide. As discussed above, phase stability depends on the grain/particle size of the sensing element (oxide crystal), the operating temperature and pressure conditions, and the presence of impurities and these are some critical factors to consider when selecting a sensor material.

Metal oxide films can be made in various ways. In one embodiment of the invention a sol-gel route is employed wherein the isopropoxide of the metals (e.g., molybdenum isopropoxide, tungsten isopropoxide, etc.) is mixed with an alkanol (e.g., propanol, butanol, etc.) to form a sol-gel of the corresponding oxide (e.g., MoO₃, WO₃, etc.). The sol-gels are amorphous colloidal solutions. Such sol-gels can be made in accordance with methods described in U.S. patent application Ser. No. 10/419,349.

In another embodiment of the invention, hybrid films of sol-gel MoO₃ and flame sprayed rutile (TiO₂) powders can be prepared.

In yet another embodiment of the invention a hydrothermal method of preparing, e.g., WO₃, chilled solutions of an alkali metal salt of the transition metal oxygenate (e.g., sodium molybdate, sodium tungstate, etc.) are acidified, preferably with a mineral acid such as hydrochloric acid, to provide the hydrated corresponding transition metal acid (e.g., molybdic acid, tungstic acid, etc). Preferably, the temperature of the alkali metal salt solution and the acid are both at the same temperature and both between 0° C. and about 20° C., preferably between 0° C. and about 10° C., and more preferably between 0° C. and about 5° C. The precipitated transition metal acid is then separated, for example by centrifugation, filtration and the like, then washed, dried and calcined to provide the crystal structure.

Orthorhombic and monoclinic phases of WO₃ can be obtained by varying the calcining temperature. The dried, calcined metal oxides are preferably mixed with a vehicle (e.g., butanol, or other alkanol) then spun coated onto a sensor substrate.

Referring now to FIG. 7, an apparatus 100 for detecting the presence of selected gaseous compounds is schematically illustrated wherein probe 110 is electrically connected to an analyzer 130 via electrical cable line 120. Probe 110 includes at least one sensor element 111 containing a transition metal oxide selected from the group consisting of monoclinic or orthorhombic WO₃, monoclinic beta-MoO₃ and UO₂. Preferably, however, the probe 110 also includes a plurality of sensor elements 112, 113, 114, 115, containing different transition metal oxides such as orthorhombic alpha-MoO₃, TiO₂, combined MoO₃-TiO₂, V₂O₅, Cr₂O₃, In₂O₃, SnO₂, MnO₂, etc. Analyzer 130 determines the electrical conductivity of each sensor element individually by conventional means. The multiple sensor elements provide a characteristic output signature for different analytes. Analyzer 130 also preferably includes pattern recognition algorithms wherein the signature of the analyte being detected is compared with known signatures stored in memory for known analytes. Various algorithms and computer controlled systems for olfactometry are known and disclosed for example in U.S. Pat. Nos. 6,411,905, 6,606,566, 6,609,068, 6,620,109, 6,839,636, 6,767,732 and 6,820,012, which are herein incorporated by reference

The following examples illustrate the invention.

EXAMPLES

Metal Oxide films of 150-200 nm thickness were prepared by deposition of the transition metal oxide on alumina substrates containing interdigitated gold or platinum electrodes. A gas flow bench was used for single sensor tests and a modified e-nose chamber for the sensor array studies.

The films were observed under SEM under room temperature condition, after heat treatments to 500° C. for different times. X-ray diffraction (XRD) patterns were recorded using a PW 1729 Philips X-ray diffractometer.

Sensing experiments were carried out in the temperature range of 200° C. to 500° C. and the gas sensitivities were plotted. The gas sensitivity S was calculated according to the following formula:

S=(R _(Air) −R _(Gas))/R _(Air)

wherein R_(Air) is the electrical resistance of the transition metal oxide film in the presence of air.

R_(Gas) is the electrical resistance of the transition metal oxide film in the presence of a gas containing the target analyte.

Example 1

A sensor was fabricated from orthorhombic alpha-MoO₃ placed in a chamber and sequentially subjected to NH₃, C₃H₆, CO, H₂, NO, and NO₂ as target analytes. The conductivity of the sensor was measured as each of the analytes was pulsed into the chamber. The results were plotted in FIG. 1, which shows the highly selective responsiveness of the alpha-MoO₃ sensor to ammonia and the flat response (non-responsiveness) of the sensor to the other target analytes. This Example demonstrates the ability of the alpha-MoO₃ to selectively detect ammonia and other nitrogen containing reducing gases in the presence of hydrocarbons, hydrogen, and oxides of carbon and/or nitrogen.

Example 2

A sensor was fabricated from orthorhombic WO₃ placed in a chamber and subjected to the target analytes as in Example 1. Surprisingly, the orthorhombic WO₃ did not respond to the presence of ammonia but did selectively respond to nitrogen oxides. Nitrogen oxide was then pulsed into the chamber at various concentrations and the conductivity of the sensor was measured. The results are shown in FIG. 2, which illustrates the sensitivity of the sensor to concentrations of nitrogen oxide as low as 1 ppm.

While not wishing to be bound by any theory, it is believed that the orthorhombic WO₃ differs in selectivity from orthorhombic MoO₃ because of the crystallographic differences. Whereas orthorhombic MoO₃ has a layered structure with a (010) basal plane built up of double chains of edge-sharing [MoO₆] octahedra connected through vertices, the orthorhombic WO₃ is structurally similar to the monoclinic beta-MoO₃. Both beta-MoO₃ and orthorhombic WO₃ are selective to nitrogen-containing oxidizing gases.

Example 3

Monoclinic WO₃ (designated as W2 and which appears as the lower graph) and orthorhombic WO₃ (designated as W3, and which appears as the upper graph) were tested for responsiveness to NO₂. FIG. 3 shows X-ray diffraction spectra for both of these polymorphs of WO₃ at room temperature. Sensors were fabricated from both polymorphs and exposed to varying concentrations of NO₂ ranging from 50 ppm to 400 ppm which were pulsed into the chamber in which the sensors were positioned. FIG. 4 shows the responsiveness to NO₂ for both the monoclinic WO₃ (upper W2 graph) and orthorhombic WO₃ (lower W3 graph). As can be seen, both polymorphs of WO₃ responded to NO₂.

Example 4

A probe was fabricated with a five sensor array containing monoclinic MoO₃ at position (1), orthorhombic MoO₃ at position (2), orthorhombic WO₃ at position (3), and two hybrid TiO₂-MoO₃ sensors, i.e., TiO₂-monoclinic MoO₃ and TiO₂-orthorhombic MoO₃ at positions (4) and (5), respectively.

The probe was tested for responsiveness to acetone and ethanol. FIG. 5A is a plot showing the pattern response of the five sensor array to acetone. The MoO₃ sensors demonstrated no sensitivity. The WO₃ sensor displayed some sensitivity and the hybrid TiO₂—MoO₃ sensors displayed sensitivity to acetone. FIG. 5B shows a pattern response of the five sensor array to exposure to ethanol. As can be seen, the MoO₃ sensors displayed no sensitivity. The WO₃ sensor exhibited a noticeable response to ethanol and the hybrid TiO₂—MoO₃ sensors exhibited great sensitivity to ethanol.

Example 5

Sensors fabricated from orthorhombic WO₃ and monoclinic MoO₃ were subjected to exposure to NO₂, NH₃ and CO at 400° C. The responses were measured and plotted in FIGS. 6A and 6B. As can be seen from FIG. 6A, the WO₃ sensor was selectively responsive to NO₂ but not NH₃ or CO. FIG. 6B shows that monoclinic MoO₃ is highly responsive to NO₂ but less responsive to NH₃ and non-responsive to CO.

While the above description contains many specifics, these specifics should not be construed as limitations of the invention, but merely as exemplifications of preferred embodiments thereof. Those skilled in the art will envision many other embodiments within the scope and spirit of the invention as defined by the claims appended hereto. 

What is claimed is:
 1. An apparatus for detecting the presence of selected gaseous compounds which comprises: a) a probe having one or more sensor elements having a conductivity which is responsive to the presence of selected gaseous compounds, at least one sensor element containing a transition metal oxide selected from the group consisting of WO₃, beta-MoO₃ and UO₂; and b) means for measuring the conductivity of each said sensor element.
 2. The apparatus of claim 1 wherein at least one sensor element contains WO₃.
 3. The apparatus of claim 2 wherein the WO₃ is monoclinic.
 4. The apparatus of claim 2 wherein the WO₃ is orthorhombic.
 5. The apparatus of claim 1 wherein the at least one sensor contains β-MoO₃.
 6. The apparatus of claim 1 comprising at least a first sensor element containing WO₃ and a second sensor element containing β-MoO₃.
 7. The apparatus of claim 6 further comprising at least a third sensor containing TiO₂—MoO₃ hybrid.
 8. The apparatus of claim 6 further comprising at least a third sensor element containing one or more oxides selected from the group consisting of V₂O₅, Cr₂O₃, In₂O₃, SnO₂ and MnO₂. 